This is a phase-II/III, randomized, two-armed comparison study to compare micronized atovaquone and azithromycin (AT/AZ) with trimethoprim-sulfamethoxazole (TMP/SMX) in preventing serious bacterial infections in HIV-infected children 3 months to 19 years of age. After stratifying for previous use of TMP/SMX and/or IVIG, subjects will be randomized to one of two arms, TMP/SMX (5 mg/kg/day) OR micronized atovaquone (30 mg/kg/day) plus azithromycin (5 mg/kg/day). Both arms will have matching placebos. Subjects will be evaluated for safety and tolerance of the study drugs. Cross-over to an alternative study-drug regimen will occur for either arm if a serious treatment-related toxicity is observed. Subjects will also be tested for equivalence of antibacterial agents by the monitoring for occurrence of serious bacterial infections or PCP breakthrough. When a serious bacterial infection endpoint (a second infection is identified) is reached, subjects will be crossed over to the alternative study-drug regimen. The first 30 subjects will have a pharmacokinetic profile performed to assist in insuring that the previously established blood concentrations of azithromycin and atovaquone are not significantly affected by the administration of the two drugs in combination.